Two-terminal type non-linear element, manufacturing method thereof and liquid crystal display panel

ABSTRACT

The present invention provides a MIM type non-linear element in which the capacitance is sufficiently small and in which little changes over time are exhibited in the current-voltage characteristics, a liquid crystal display panel with high image quality using the MIM type non-linear element, and a method of manufacturing the MIM type non-linear element. The MIM type non-linear element includes a first conductive film, an insulation film and a second conductive film, which are laminated on a substrate. The insulation film has a relative dielectric constant of 25.5 or less, preferably 24.0-25.5. In elementary analysis by SIMS, a hydrogen spectrum of the boundary region between the first conductive film and the insulation film has a width of 10 nm or more in the depth direction at an intensity of one tenth of the peak intensity. The first conductive film of the MIM type non-linear element shows a peak temperature of 300° C. or higher in a thermal desorption spectroscopy of hydrogen. The MIM type non-linear element is manufactured by, for example, a method containing the steps of (a) forming the first conductive film, (b) heat-treating the first conductive film at a temperature of 300° C. or higher in an inert gas, (c) forming the insulation film on the surface of the first conductive film by anodization of the first conductive film, and (d) forming the second conductive film on the surface of the insulation film.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to a two-terminal type non-linear elementused as a switching element, a method of manufacturing the two-terminaltype non-linear element, and a liquid crystal display panel containingthe two-terminal type non-linear element.

2. Description of Related Art

In an active matrix type liquid crystal display device, the spacebetween an active matrix substrate containing switching elementsprovided for respective pixel regions to form a matrix array, and anopposite substrate containing, for example, color filters, is filledwith a liquid crystal, and the alignment state of the liquid crystal ineach of the pixel regions is controlled to display predetermined imagedata. For each of the switching elements, a threeterminal element suchas a thin film transistor (TFT), or a two-terminal type element such asa metal-insulator-metal (MIM) non-linear element (referred to as a “MIMelement” hereinafter) is generally used. The switching element havingthe two-terminal type element is excellent in that no cross-over shortcircuiting occurs, and the manufacturing process can be simplified, ascompared with the three-terminal element.

In a liquid crystal display device containing the MIM elements, in orderto realize a liquid crystal display panel which exhibits high imagequality and high contrast in which unevenness in the display, afterimage phenomenon and sticking image phenomenon are not observed, it isimportant to satisfy the following conditions for the characteristics ofthe MIM element:

(1) The capacitance of the MIM element is sufficiently smaller than thatof the pixel of the liquid crystal display panel;

(2) Changes in the current-voltage characteristics of the MIM elementwith respect to time are sufficiently small;

(3) The current-voltage characteristics of the MIM element have goodsymmetry;

(4) The current-voltage characteristics of the MIM element havesufficiently high steepness; and

(5) The element resistance of the MIM element is sufficiently uniform ina wide voltage range.

Namely, in order to increase the contrast, it is necessary that thecapacitance of the MIM element be sufficiently small, as compared withthe capacitance of the liquid crystal display panel which containscapacitances of one pixel electrode, a liquid crystal which is providedin the region driven by the pixel electrode, and a signal line providedopposite to the pixel electrode.

Also, in order to increase the contrast, the current-voltagecharacteristics of the MIM element should have sufficiently highsteepness. In order to make the unevenness in the displayunrecognizable, it is necessary for the MIM element to have sufficientlyuniform resistance in a wide voltage range. In order to make the afterimage unrecognizable, it is necessary for the MIM element to showsufficiently small changes in the current-voltage characteristics withrespect to time. Further, in order to make the sticking imageunrecognizable, it is necessary for the MIM element to exhibitcurrent-voltage characteristics having sufficiently small changes withrespect to time and having good symmetry.

The “after image” is the phenomenon in which a first displayed image isobserved when the display screen is switched to display a second image,and the first displayed image disappears in a short time. The “imagesticking” is the phenomenon in which a first image is displayed over along period of time, the display screen is switched to display a secondimage, and the first image is observed for a long time. The observedfirst image of the latter phenomenon will be referred to as the“sticking image”. Further, the phrase “the current-voltagecharacteristics have good symmetry” means that there is a sufficientlysmall difference between the absolute values of currents at a givenvoltage when a current is passed from a first conductive film to asecond conductive film and when a current is passed from the secondconductive film to the first conductive film.

Types of MIM elements have previously been proposed. For example,Japanese Patent Unexamined Publication No. 52-149090 proposes an MIMelement containing a first conductive film of tantalum, an insulationfilm containing a metal oxide film formed by anodization of the firstconductive film, and a second conductive film of chromium formed on thesurface of the insulation film. Since the insulation film is formed byanodization of the surface of the first conductive film, the insulationfilm is formed with a uniform thickness without pinholes. JapanesePatent Unexamined Publication No. 57-122478 proposes that a diluteaqueous solution of citric acid is used as electrolyte for anodization.In these techniques, the characteristics (2) to (5) of the MIM elementare not always sufficiently good. Namely, the changes with respect totime, the symmetry and the steepness of the current-voltagecharacteristics are insufficient, and the element resistance is notsufficiently uniform in a wide voltage range. Therefore, a liquidcrystal display panel containing the MIM elements has a problem in thatit is difficult to maintain high contrast in a wide temperature range inthe panel, and it easily brings about unevenness in the display.

International Patent Application PCT/JP94/00204 (International Laid-OpenNo. WO94/18600) proposes a structure in which a tantalum alloy filmcontaining tungsten is used as a first conductive film of an MIMelement. In this technique, since the first conductive film of the MIMelement contains not a single tantalum film but an alloy film ofspecific elements such as tantalum and tungsten, the characteristics (2)and (3), i.e., the changes with respect to time and symmetry of thecurrent-voltage characteristics of the MIM element, are improved ascompared with the techniques previously discussed with respect toJapanese Patent Unexamined Publication Nos. 52-149090 and 57-122478. Inaddition, the after image can be decreased to an unrecognizable level,and the contrast can be kept high in a wide temperature range. However,this technique does not provide sufficiently high contrast at hightemperatures, and, as such, cannot be used in applications requiringhigh contrast at high temperatures.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a two-terminal typenon-linear element exhibiting the characteristics (1) to (5) requiredfor the MIM element described above. In particular, the MIM elementshould exhibit steepness of the current-voltage characteristics,sufficiently small changes in the current-voltage characteristics withrespect to time, and high reliability.

Another object of this invention is to provide a liquid crystal displaypanel containing the two-terminal type non-linear element, exhibitinghigh contrast and high image quality, and causing neither unevenness norsticking image in the display.

A further object of the present invention is to provide a method ofmanufacturing the two-terminal type non-linear element having the abovedescribed excellent characteristics.

In accordance with the present invention, a two-terminal non-linearelement (referred to as a “MIM type non-linear element” hereinafter)contains a first conductive film, an insulation film and a secondconductive film, which are laminated on a substrate. The insulation filmis obtained by anodization of the first conductive film in a electrolytecontaining a water solution. The insulation film has a relativedielectric constant of 25.5 or less, and preferably a relativedielectric constant of 24.0-25.5.

In an MIM type non-linear element of the present invention, a hydrogenspectrum of the boundary region between the first conductive film andthe insulation film is obtained by elementary analysis carried out bysecondary ion-mass spectrography (SIMS) using irradiation of cesiumprimary ions. The hydrogen spectrum in the depth direction preferablyhas a width of 10 nm or more, more preferably a width of 15-50 nm, at anintensity of one tenth of the peak intensity.

In accordance with the present invention, in an MIM type non-linearelement, a thermal desorption spectroscopy of the first conductive filmpreferably has a peak temperature of the hydrogen spectrum of 300° C. orhigher, more preferably a peak temperature of 300-400° C.

In the MIM type non-linear element of the present invention, the secondconductive film is not limited to a metallic film, and includestransparent conductive films of ITO and the like.

In an MIM type non-linear element of the present invention, inparticular, the capacitance of the MIM type non-linear is sufficientlysmall, and the steepness of the voltage-current characteristic is large.In addition, in the MIM type non-linear element of the presentinvention, it is possible to have extremely small changes involtage-current characteristics over the passage of time. Also, the MIMtype non-linear element of the present invention can maintain highreliability over a long period of time. In the MIM type non-linearelement of the present invention, the insulation film is possiblydivided into three layers including upper and lower semiconductor layersadjacent to the first and second conductive films, and an insulatorlayer formed between the two semiconductor layers, with the insulatorlayer having a band gap larger than that of the semiconductor layers.One possible reason for the changes in the current-voltagecharacteristics with respect to time is that the fine crystal of theinsulator layer is gradually broken by the application of a voltage. Inthe MIM type non-linear element of the present invention, thesemiconductor layer containing hydrogen and having a specified thicknessis present between the insulator layer and the conductive films,particularly the first conductive film, thereby decreasing the effectivevoltage applied to the insulator layer. As a result, changes in thecurrent-voltage characteristics with respect to time are possiblydecreased.

In accordance with the present invention, a method of manufacturing aMIM type non-linear element contains the steps of:

(a) forming a first conductive film on a substrate;

(b) heat-treating the first conductive film at a temperature of 300° C.or higher in an inert gas;

(c) forming an insulation film on the surface of the first conductivefilm by anodization of the first conductive film; and

(d) forming a second conductive film on the surface of the insulationfilm.

This manufacturing method is capable of obtaining the MIM typenon-linear element of the present invention by carrying out a heattreatment step.

In accordance with the present invention, a liquid crystal display panelcontains the MIM type non-linear element. More specifically, the liquidcrystal display panel includes a first transparent substrate with signallines arranged in a predetermined pattern on the first transparentsubstrate, MIM type non-linear elements connected to the signal lines,and pixel electrodes connected to the MIM type non-linear elements. Theliquid crystal display panel a further includes a second substratecontaining other signal lines provided opposite to the pixel electrodes,and a liquid crystal layer sealed between the first substrate and thesecond substrate. This liquid crystal display panel has high contrastand causes less image sticking, and thus can display high-qualityimages. The liquid crystal display can be used for a wide range ofapplications.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view showing a principal portion of a liquid crystaldisplay panel to which a MIM type non-linear element of the presentinvention is applied;

FIG. 2 is a cross-sectional view taken along line A—A in FIG. 1;

FIG. 3 is a cross-sectional view showing another example of theconfiguration of a MIM type non-linear element of the present invention;

FIG. 4 shows in schematic MIM type non-linear elements in a circuit of aliquid crystal display panel of the present invention;

FIG. 5 is a perspective view with portions of a liquid crystal displaypanel shown in section;

FIG. 6 is a plan view showing main elements of a liquid crystal displaypanel in which an MIM type non-linear element of the present inventionhaving a back-to-back structure is applied;

FIG. 7 is a cross-sectional view taken along line B—B in FIG. 6;

FIG. 8 shows a SIMS spectrum determined for an MIM type non-linearelement relating to an example of the present invention;

FIG. 9 shows a SIMS spectrum determined for an MIM type non-linearelement relating to a comparative example of the present invention;

FIG. 10 shows a SIMS spectrum determined for an MIM type non-linearelement relating to an example of the present invention;

FIG. 11 shows a SIMS spectrum determined for an MIM type non-linearelement relating to an example of the present invention;

FIG. 12 shows a thermal desorption spectroscopy determined for a firstconductive film of an MIM type non-linear element relating to an exampleof the present invention;

FIG. 13 shows a thermal desorption spectroscopy determined for a firstconductive film of an MIM type non-linear element relating to acomparative example of the present invention;

FIG. 14 shows in schematic an apparatus for determining the thermaldesorption spectroscopy;

FIG. 15 shows in schematic a sample for obtaining the thermal desorptionspectroscopy;

FIG. 16 shows the relationship between the current value and the shiftvalue when a fixed voltage is applied to an MIM type non-linear element;

FIG. 17 shows the relationship between the pre-annealing temperature,the electrostatic capacitance and the relative dielectric constant,determined for an MIM type non-linear element;

FIG. 18 shows the relationship between the temperature and the currentvalue when a fixed voltage is applied to an MIM type non-linear elementrelating to an example of the present invention; and

FIG. 19 shows the relationship between the temperature and the currentvalue when a fixed voltage is applied to an MIM type non-linear elementrelating to a comparative example of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be described belowwith reference to the drawings.

FIG. 1 is a plan view showing in schematic one unit of a liquid crystaldriving electrode containing the MIM type non-linear element of thepresent invention. FIG. 2 is a schematic sectional view taken along lineA—A in FIG. 1.

A MIM type non-linear element 20 includes a first substrate 30 havinginsulation capability and transparency which may be, for example, aglass or plastic substrate. MIM type non-linear element 20 furtherincludes an insulation film 31 formed on the surface of the substrate30, a first conductive film 22 containing tantalum or a tantalum alloy,an insulation film 24 formed on the surface of the first conductive film22 by anodization, and a second conductive film 26 formed on the surfaceof the insulation film 24. The first conductive film 22 of the MIM typenon-linear element 20 is connected to a signal line (scanning line) 12,and the second conductive film 26 is connected to a pixel electrode 34.

The insulation film 31 contains, for example, tantalum oxide. Since theinsulation film 31 is formed for preventing peeling of the firstconductive film 22 due to heat treatment carried out after deposition ofthe second conductive film 26, and for preventing diffusion ofimpurities into the first conductive film 22 from the substrate 30, theinsulation film 31 is not necessarily required in cases where theseproblems do not occur.

The first conductive film 22 may be a single tantalum film or an alloyfilm containing tantalum as a main component and an element in GroupsVI, VII or VIII in the Periodic Table. Preferable examples of theelement added to the alloy include tungsten, chromium, molybdenum,rhenium, yttrium, lanthanum, dysprosium, and the like. Tungsten is morepreferable as the element, and the content thereof is preferably 0.1-6atomic %.

The first conductive film 22 preferably shows a peak temperature of 300°C. or higher, more preferably 300-400° C., in a thermal desorptionspectroscopy of hydrogen. The method of measuring the thermal desorptionspectroscopy will be described in detail later.

The insulation film 24 is formed by anodization in a electrolytecontaining a water solution. Also, prior to anodization, after formingthe first conductive film 22, as will be described later, heat-treatingis carried out at a predetermined temperature. As a result, it ispossible to make the relative dielectric constant of the insulation film24 smaller. This relative dielectric constant is set at 25.5 or less,preferably at 24.0-25.5.

Although the composition of the second conductive film 26 is notlimited, the second conductive film 26 generally contains chromium. Thepixel electrode 34 includes a transparent conductive film such as an ITOfilm or the like.

The present invention is characterized in that the hydrogen spectrum ofthe boundary region between the first conductive film 22 and theinsulation film 24 in the depth direction has a width of 10 nm or more,preferably 15‥50 nm, at an intensity Ih which is one tenth of the peakintensity Ip. The intensity is determined by the count number ofsecondary ions, which is indicated in logarithmic scale. This hydrogenspectrum is obtained by elementary analysis carried out by secondaryion-mass spectrography (SIMS) using irradiation of cesium primary ions.Namely, it is important that hydrogen is present in the specified regionnear the boundary between the first conductive film 22 and theinsulation film 24.

As shown in FIG. 3, the second conductive film and the pixel electrodemay include the same transparent conductive film 36. By forming thesecond conductive film and the pixel electrode by using a single film,the number of the manufacturing steps required for forming the films canbe decreased.

FIG. 4 shows an example of the circuit of active matrix type liquidcrystal display panels containing the MIM type non-linear elements 20.The liquid crystal display panel 10 shown in FIG. 4 includes a scanningsignal driving circuit 100 and a data signal driving circuit 110. In theliquid crystal display panel 10, a plurality of signal lines, i.e.,scanning lines 12 and data lines 14, are provided so that the scanninglines 12 and the data lines 14 are driven by the scanning signal drivingcircuit 100 and the data signal driving circuit 110, respectively. Ineach of pixel regions 16, the MIM type non-linear element 20 and aliquid crystal display element (a liquid crystal layer) 25 are connectedin series between a scanning line 12 and a data line 14. Although, inFIG. 4, the MIM type non-linear element 20 is connected to the scanningline 12 side and the liquid crystal display element 25 is connected tothe data line 14 side, conversely, the MIM type non-linear element 20and the liquid crystal display element 25 may be provided on the dataline 14 side and the scanning line 12 side, respectively.

FIG. 5 is a perspective view showing an example of the structure of theliquid crystal display panel in accordance with an embodiment of thepresent invention. The liquid crystal display panel 10 includes twosubstrates, i.e., a first substrate 30 and a second substrate 32, whichoppose each other, and a liquid crystal sealed between the substrates 30and 32 to form the liquid crystal display element 25 shown in FIG. 4. Onthe surface of the insulating film 31 are provided the plurality ofsignal lines (scanning lines) 12. On the second substrate 32 theplurality of signal lines (data lines) 14 are formed in strips to crossthe scanning lines 12. Further, the pixel electrodes 34 are connected tothe scanning lines 12 through the MIM type non-linear elements 20.

On the basis of the signals applied to the scanning lines 12 and thedata lines 14, the liquid crystal display element 25 is switched to adisplay state, non-display state or intermediate state to control thedisplay operation. As the method of controlling the display operation, ageneral method can be used.

FIGS. 6 and 7 show a MIM type non-linear element in accordance withanother embodiment of the present invention. FIG. 6 is a plan viewschematically showing one unit of a liquid crystal driving electrodecontaining a MIM type non-linear element and FIG. 7 is a sectional viewschematically showing the portion taken along line B—B in FIG. 6.

The MIM type non-linear element 40 shown in FIGS. 6 and 7 is differentfrom the MIM type non-linear element 20 shown in FIGS. 1 and 2, forexample, in that it has a back-to-back structure. Namely, the MIM typenon-linear element 40 has a structure in which a first MIM typenon-linear element portion 40 a and a second MIM type non-linear elementportion 40 b are connected in series with opposite polarities.

Specifically, the MIM type non-linear element 40 contains a substrate 30having insulation capability and transparency which may be, for example,a glass or plastic substrate. The MIM type non-linear element 40 furthercontains an insulation film 31 formed on the surface of the substrate30, a first conductive film 42 containing tantalum or a tantalum alloy,an insulation film 44 formed on the surface of the first conductive film42 by anodization, and two second conductive film portions 46 a and 46 bof the first and second MIM type non-linear element portions 40 a and 40b, respectively, formed on the surface of the insulation film 44 apartfrom each other. The second conductive film portion 46 a of the firstMIM type nonlinear element portion 40 a is connected to a signal line (ascanning line or data line) 48, and the second conductive film portion46 b of the second MIM type non-linear element portion 40 b is connectedto a pixel electrode 45. The thickness of the insulation film 44 is setto be smaller than that of the insulation film 24 of the MIM typenon-linear element 20 shown in FIGS. 1 and 2, for example, by about ahalf thereof.

Since the specified characteristics and constructions of the componentsof the MIM type non-linear element 40, such as the first conductive film42, the insulation film 44 and the second conductive film portions 46 aand 46 b, are same as those of the MIM type non-linear element 20disclosed above, description thereof is omitted.

The MIM type non-linear element 20 shown in FIG. 2, for example ismanufactured by, for example, the following process:

First, the insulating film 31 containing tantalum oxide is formed on thesubstrate 30. The insulating film 31 can be formed by a method ofthermally oxidizing a tantalum film deposited by, for example,sputtering or a method of sputtering or cosputtering using a tantalumoxide target. The insulation film 31 is provided for making adhesion ofthe first conductive film 22 closer to the substrate 30, and preventingimpurity diffusion into the first conductive film 22 from the substrate30. The insulation film 31 is formed to a thickness of 50-200 nm, forexample.

On the insulation film 31, the first conductive film 22 is formedcontaining tantalum or a tantalum alloy. The thickness of the firstconductive film 22 is appropriately selected according to application ofthe MIM type non-linear element, and is generally about 100-500 nm. Thefirst conductive film can be formed by sputtering or electron beamdeposition. With regard to the method of forming the first conductivefilm 31 containing a tantalum alloy, a sputtering or cosputtering methodusing a mixed target or an electron beam deposition method can be used.With regard to the element contained in the tantalum alloy, elements inGroups VI, VII or VIII in the Periodic Table, preferably elements suchas tungsten, chromium, molybdenum, rhenium and the like, can beselected.

The first conductive film 22 is patterned by photolithography andetching, which are generally used. The signal lines (scanning lines) 12are formed by the same step as the step of forming the first conductivefilm 22.

Next, heat treatment of the formed wafer is carried out at a temperatureof 300° C. or higher, preferably 300-400° C., in an atmosphere of aninert gas, e.g., a rare gas such as argon or the like, or nitrogen gas.When the heat treatment temperature exceeds 400° C., an adverse effectis produced in the glass or plastic substrate of the wafer. Although thetime required for heat treatment depends upon the thickness of the firstconductive film, the heat capacitance of the annealing furnace, thenumber of the wafers treated, the thickness of the glass or plasticsubstrate of a wafer, the set temperature, etc., the time of about 10 to120 minutes is used, for example.

By carrying out this heat treatment, as described above, it is possibleto make the relative dielectric constant of the insulation film 24smaller in comparison to a case when heat treatment is not carried out.Specifically, it is possible to set the relative dielectric constant to25.5 or less. Further, this heat treatment enables distribution ofhydrogen with a specified width (film thickness) of preferably 10 nm ormore in the boundary region between the first conductive film 22 and theinsulation film 24, and enables control of the hydrogen desorptiontemperature in the first conductive film 22.

Next, the surface of the first conductive film 22 is oxidized byanodization to form the insulation film 24. At the same time, thesurfaces of the signal lines (scanning lines) 12 are oxidized to forminsulation films. The preferable thickness of the insulation film 24 isselected according to application, and is, for example, about 20-70 nm.Although the composition of the chemical solution (electrolyte) used foranodization is not particularly limited, 0.01-0.1% by weight of citricacid aqueous solution, for example, can be used.

Next, a metallic film of chromium, aluminum, titanium, molybdenum or thelike is deposited by sputtering, for example, to form the secondconductive film 26. The second conductive film is formed to a thicknessof, for example, 50-300 nm, and then patterned by photolithography andetching, which are generally used. Then, an ITO film is deposited to athickness of 30-200 nm by sputtering or the like, followed byphotolithography and etching, which are generally used, to form thepixel electrodes in a predetermined pattern.

In the MIM type non-linear element 20 shown in FIG. 3, the secondconductive film and the pixel electrode is made of the transparentconductive film 36 such as the same ITO film or the like. In this case,since the second conductive film and the pixel electrode can be formedin the same step, the manufacturing process can further be simplified.

EXAMPLES

The present invention will be described in further detail below withreference to examples and comparative examples.

Example 1

In this example, a MIM type non-linear element having the back-to-backstructure shown in FIGS. 6 and 7 was used. Specifically, a tantalum filmcontaining 0.2 atomic % of tungsten was deposited to a thickness of 150nm on a glass substrate by sputtering, and then patterned to form afirst conductive film. Next, heat treatment (pre-annealing) was carriedout at 350° C. for 30 minutes in a nitrogen atmosphere. Next, constantcurrent electrolysis was carried out by using a 0.05 weight % of citricacid aqueous solution as electrolyte with a current density of 0.4mA/cm² until a voltage of 15V was obtained. This step was followed byanodization of the tantalum film. As a result, a tantalum oxide filmhaving a thickness of about 30 nm was formed.

Further,.heat treatment was carried out at 320° C. for 30 minutes in anitrogen atmosphere, followed by cooling in an air atmosphere tostabilize the anodized film (insulation film). Then, chromium wasdeposited to 100 nm on the insulation film by sputtering, and thenpatterned to form a second conductive film. In the first conductivefilm, a portion which constitute signal lines is separated tomanufacture a MIM type non-linear element.

Comparative Example 1

A MIM type non-linear element was manufactured by the same method asExample 1 except that heat treatment (pre-annealing) in Example 1 wasnot carried out.

Examples of the experiments carried out for the MIM type non-linearelements of Example 1 and Comparative Example 1 will be described below.

(a) SIMS

Example 1

FIG. 8 shows the results of SIMS by cesium ion etching, which wascarried out to determine a profile of each of the elements (¹⁸O, C, Hand Ta) contained in the insulation film and the first conductive film.In FIG. 8, the depth from the surface of the insulation film in thefirst conductive film and the insulation film is shown on the horizontalaxis, and the count number of secondary ions is shown in logarithmicscale on the vertical axis. In FIG. 8, a line shown by character “h”passes through the peak of the hydrogen spectrum, and shows the boundarybetween the first conductive film and the insulation film forconvenience.

It was confirmed by FIG. 8 that in this example, the hydrogen spectrumof the boundary region between the first conductive film (Ta) and theinsulation film (TaO_(x)) in the depth direction shows a relativelygentle peak having a bell-like form. The width at an intensity Ih of onetenth of the peak intensity Ip is about 17 nm. In SIMS measurement, ifthe anodized film (insulation film) of a sample is excessively thin,data of the boundary between the insulation film and the firstconductive film is hardly obtained, and thus the thickness of theanodized film is set to about 70 nm. It was also confirmed that theboundary state of the anodized insulation film does not changeregardless of change in the thickness of the anodized film.

Comparative Example 1

FIG. 9 shows the results of the same SIMS carried out for ComparativeExample 1. It was confirmed by FIG. 9 that in this example, in ahydrogen spectrum of the boundary region between the first conductivefilm (Ta) and the insulation film (TaO_(x)) in the depth direction, thepeak has an acute triangular shape, and the width at an intensity Ih ofone tenth of the peak intensity Ip is about 7 nm.

Thermal Dependency of the Hydrogen Peak

In order to confirm what kind of influence the annealing temperature hason the hydrogen peak in SIMS, samples differing only in annealingtemperature were prepared to obtain a spectrum according to SIMS. Thesamples were prepared the same way as in Example 1 except that theannealing temperature was set at 300° C. and 400° C. The SIMS spectrumobtained with these samples is shown in FIGS. 10 and 11, respectively.

FIG. 10 shows the spectrum of samples obtained with the annealingtemperature set at 300° C. From FIG. 10, the hydrogen spectrum of thesesamples in this example was confirmed to have a width in the depthdirection of approximately 12 nm at an intensity Ih of one tenth of thepeak intensity Ip.

FIG. 11 shows the spectrum of the samples obtained when the annealingtemperature is set at 400° C. From FIG. 11, it is confirmed that in thisexample, the hydrogen spectrum of the samples has a width in the depthdirection of approximately 23 nm at an intensity Ih of one tenth of thepeak intensity Ip.

From FIGS. 8-11, it is confirmed that the hydrogen peak width increaseswith the rise in annealing temperature.

(b) Thermal Desorption Spectroscopy

Example 1

Measurement of the first conductive film (Ta) by the thermal desorptionspectral (TDS) method will be described below. This measurement wascarried out by using the thermal desorption spectral measurementapparatus shown in FIG. 14.

Referring to FIGS. 14 and 15, the thermal desorption spectralmeasurement apparatus 500 contains a quadrupole mass spectrometer 502and an infrared heater 504 which are provided in a vacuum chamber 510.The back side of a sample 520 is heated by the infrared heater 504, andgases generated from the sample 520 are measured by the quadrupole massspectrometer 502 to obtain a thermal desorption spectroscopy. Thetemperature of the sample was controlled by using a thermocouple TC1 onthe back side of the sample 520 from the viewpoint of controllability.In order to measure the surface temperature of the sample 520, athermocouple TC2 was also provided on the face side of the sample 520.The quartz substrate 522 used for the sample 520 was poor in thermalconductivity and had a thickness of as large as 1.1 mm, thereby causinga difference between the temperatures of the thermocouples TC1 and TC2.However, in the actual process for manufacturing a MIM type non-linearelement, the temperature of the thermocouple TC1 is substantially thesame as the temperature of the thermocouple TC2. In TDS measurement,quartz glass is used as a substrate. This is because the heat resistanttemperature of the substrate is increased for measurement up to a hightemperature of 1000° C. It is confirmed that even if the substrate ischanged to a usual no-alkali glass, the current-voltage characteristicsof the MIM type non-linear element are the same.

As shown in FIG. 15, the sample 520 used in measurement has a tantalumfilm 524 containing 0.2 atomic % of tungsten formed to a thickness of200 nm on the quartz substrate 522 having a thickness of 1.1 mm. Thetantalum film is formed on the quartz substrate by sputtering, and thensubjected to the pre-annealing discussed above. Namely, the laminateshown in FIG. 15 was pre-annealed by heating to 350° C. in a nitrogenatmosphere and maintained at 350° C. for 30 minutes in a nitrogenatmosphere in a heat treatment furnace, and then cooled to 200° C. at arate of 1.0° C. /min. The laminate was then taken out from the heattreatment furnace and used as a sample for thermal desorption spectralmeasurement.

A thermal desorption spectroscopy was measured by using the sample 520.The results are shown in FIG. 12. In FIG. 12, the temperature of thecontrol thermocouple TC1 is shown on the horizontal axis, and theintensity as the measured value of a gas at a molecular mass of 2 amucorresponding to hydrogen gas (H₂) is shown on the vertical axis. In thespectrum shown in FIG. 12, a peak P1 was obtained. As described above,since the temperature of the thermocouple TC1 is different from that ofthe thermocouple TC2, the surface temperature of the sample 520 at thepeak P1 measured by the thermocouple TC2 was about 400° C. The lowerlimit temperature of the spectrum measured by the thermocouple TC2 wasabout 300° C.

Further, the number of hydrogen atoms determined by the integralintensity over the whole temperature region of the spectrum was1.76×10¹⁶/cm².

Comparative Example 1

Comparative Example 1 was examined. FIG. 13 shows the results of thesame thermal desorption spectroscopy as Example 1. In the spectrum shownin FIG. 13, peak P2 was obtained. The temperature at peak P2 measured bythe thermocouple TC2 was about 290° C.

Further, the number of hydrogen atoms determined by the integralintensity over the whole temperature region of the spectrum was1.16×10¹⁶/cm².

(c) Shift Value

Example 1

In order to examine changes with respect to time in the current-voltagecharacteristics of the MIM type non-linear element of Example 1, a shiftvalue as an index of changes was determined. As a result, the shiftvalue was determined to be 2%. This shift value is defined as a valueI_(s) represented by the following equation when a rectangular wavevoltage is applied to the MIM type non-linear element with the polaritychanged at intervals of one second. At this time, the applied voltage isset so that a current of 1×10⁻⁷ A flows through one pixel of the liquidcrystal display panel.

I _(s)={(I ₁₀₀ −I ₀)/I ₀}100(%)

In this equation, I₀ indicates the absolute value of the initial current(1 second), and I₁₀₀ indicates the absolute value of the current 100seconds after. In practical use, in order to prevent image sticking, theshift value is preferably within the range of −5 to +5%, more preferablywithin the range of −2 to +2%.

Comparative Example 1

The shift value of Comparative Example 1 determined in the same manneras described above in Example 1 was 4%.

Pre-annealing Dependency of the Shift Value

A plurality of samples of the present invention which were pre-annealed,and a plurality of comparative samples which were not pre-annealed werefurther formed, and shift values of these samples were determined. Theresults obtained are shown in FIG. 16.

In FIG. 16, the logarithmic scale of the current passing through the MIMtype non-linear element when a voltage of 4 V was applied is shown onthe horizontal axis, and the shift value is shown on the vertical axis.It was clearly confirmed by FIG. 16 that both the shift value and theholding current required for obtaining a sufficient contrast aresatisfied by pre-annealing. “The holding current required for obtaininga sufficient contrast” is a current value for preventing a signal inputin the selection period from varying in a holding period. In regard tothe holding current, the current flowing through the MIM type non-linearelement is preferably 1×10⁻¹¹ A or less, more preferably 4×10⁻¹² A orless, for practical use, for example, when a voltage of 4 V is appliedto the element.

(d) Non-linear Coefficient

Example 1

The current-voltage characteristics of the MIM type non-linear elementof Example 1 were measured, and the non-linear coefficient (β value)indicating steepness was computed. As a result, the β value was 4.7.

Comparative Example 1

The β value of Comparative Example 1 determined in the same manner asExample 1 was 4.7.

(e) Relative Dielectric Constant

Example 1

The relative dielectric constant was determined from the electrostaticcapacitance and the film width of the insulation film of an MIM typenon-linear element. Specifically, the measurement of the electrostaticcapacitance is determined by connecting 2,500 MIM type non-linearelements measuring 4 μm on each side, in parallel, and applying analternating current having an effective voltage of 1 V and a frequencyof 10 kHz. The electrostatic capacitance of the samples of the MIM typenon-linear elements of Example 1 was 305 pF. Further, the film width ofthe insulation film measured using an ellipsometer was found to be 28.8nm. Also, the relative dielectric constant of the insulation film, ascomputed from the electrostatic capacitance and film width obtained, was24.8.

Comparative Example 1

The relative dielectric constant of Comparative Example 1 determined inthe same manner as Example 1 was 25.7.

Annealing Temperature Dependency of the Relative Dielectric Constant

In order to find the dependency of the annealing temperature of therelative dielectric constant, a plurality of samples were produced inwhich the pre-annealing temperature was altered. The electrostaticcapacitance, and the relative dielectric constant computed from thiselectrostatic capacitance and the film width of the insulation film,were determined for these samples. These results are shown in FIG. 17.In FIG. 17, a plurality of points plotted on the same pre-annealingtemperature correspond to the plurality of samples produced underidentical conditions.

From FIG. 17, it can be understood that with a pre-annealing temperatureof 300-400° C., the relative dielectric constant becomes approximately24.5-25.5. Further, even if the pre-annealing temperature increasesabove 400° C., it was confirmed that the relative dielectric constantdoes not fall below 24.0.

(f) Temperature Dependency of Current Value

Example 1

Using the samples obtained in Example 1, the current value flowing tothe MIM type nonlinear elements was measured by changing the temperatureof the substrate. The results are shown in the Arrhenius plotting ofFIG. 18. In FIG. 18, the horizontal axis denotes the reciprocal((1/T)×1000) of the absolute temperature, and the vertical axis denotesthe current value. Also, the line denoted by symbol “a” is the datacorresponding to a writing voltage when 10 volts is applied, and theline denoted by symbol “b” is the data corresponding to a writingvoltage when 4 volts is applied.

When the slopes of line a and line b are determined by FIG. 18, they arefound to be, respectively, −6.5792 and −7.3043, both of which are fairlyclose. As a result, because the temperature dependency of the currentvalue has substantially the same inclination in writing voltage andholding voltage, deterioration of the on/off characteristic is confirmedto be slight.

Comparative Example 1

Based on the samples obtained in Comparative Example 1, the temperaturedependency of the current value was determined in the same manner as inExample 1. The results are shown in FIG. 19. In FIG. 19, the linedenoted by symbol “c” is the data corresponding to the writing voltagewhen 10 volts is applied, and the line denoted by symbol “d” is the datacorresponding to the holding voltage when 4 volts is applied.

When the slopes of lines c and d are determined in FIG. 19, they arefound respectively to be −6.5066 and −7.7149. These are fairly differentfrom each other as compared with Example 1. Therefore, it can beunderstood that when considering points such as the on/offcharacteristic, this is inferior to Example 1.

As a result of manufacture of a liquid crystal display panel by usingthe MIM type non-linear element of Example 1, a contrast ratio of 100:1or more could be obtained, and no unevenness in the display wasobserved.

What is claimed is:
 1. A two-terminal type non-linear element comprisinga first conductive film, an insulating film and a second conductive filmwhich are laminated on a substrate, wherein said insulation film isobtained by anodization of said first conductive film in an electrolytecomprising a water solution after said first conductive film isheat-processed, and said insulation film has a relative dielectricconstant of 25.5 or less, and wherein hydrogen is present in a specifiedregion near a boundary between the first conductive film and theinsulation film.
 2. A two-terminal type non-linear element in accordancewith claim 1, wherein the relative dielectric constant of saidinsulation film is 24.0-25.5.
 3. A two-terminal type non-linear elementin accordance with claim 1, wherein elementary analysis is carried outby secondary ion-mass spectrography (SIMS) using irradiation of cesiumprimary ions, and wherein a hydrogen spectrum of a boundary regionbetween the first conductive film and the insulation film is obtainedhaving a width of 10 nm or more in a depth direction at an intensity ofone tenth of a peak intensity.
 4. A two-terminal type non-linear elementin accordance with claim 3, wherein the width of the hydrogen spectrumis 15-50 nm in the depth direction.
 5. A two-terminal type non-linearelement in accordance with claim 1, wherein a thermal desorptionspectroscopy of the first conductive film has a peak temperature of ahydrogen spectrum of at least 300° C.
 6. A two-terminal type non-linearelement in accordance with claim 5, wherein the peak temperature of thehydrogen spectrum is 300-400° C.
 7. A two-terminal type non-linearelement in accordance with claim 1, wherein the second conductive filmcomprises a first portion and a second portion.
 8. A two-terminal typenon-linear element in accordance with claim 7, wherein the first portionand the second portion of the second conductive film are formed apartfrom each other.
 9. A liquid crystal display panel, comprising: a firsttransparent substrate comprising first signal lines disposed in apredetermined pattern on the first transparent substrate, a two-terminaltype non-linear element connected to the first signal lines, and pixelelectrodes connected to the two-terminal type non-linear element; asecond transparent substrate comprising second signal lines positionedopposite to the pixel electrodes; and a liquid crystal layer sealedbetween the first transparent substrate and second transparentsubstrate; wherein the two-terminal type non-linear element comprises afirst conductive film, an insulating film and a second conductive filmwhich are laminated on the first transparent substrate, and wherein saidinsulation film is obtained by anodization of said first conductive filmin an electrolyte comprising a water solution after said firstconductive film is heat-processed, and said insulation film has arelative dielectric constant of 25.5 or less, and wherein hydrogen ispresent in a specified region near a boundary between the firstconductive film and the insulation film.
 10. A liquid crystal displaypanel in accordance with claim 9, wherein the relative dielectricconstant of said insulation film is 24.0-25.5.
 11. A liquid crystaldisplay panel in accordance with claim 9, wherein elementary analysis iscarried out by secondary ion-mass spectrography (SIMS) using irradiationof cesium primary ions, and wherein a hydrogen spectrum of a boundaryregion between the first conductive film and the insulation film isobtained having a width of 10 nm or more in a depth direction at anintensity of one-tenth of a peak intensity.
 12. A liquid crystal displaypanel in accordance with claim 11, wherein the width of the hydrogenspectrum is 15-50 nm in the depth direction.
 13. A liquid crystaldisplay panel in accordance with claim 9, wherein a thermal desorptionspectroscopy of the first conductive film has a peak temperature of ahydrogen spectrum of at least 300° C.
 14. A liquid crystal display panelin accordance with claim 13, wherein the peak temperature of thehydrogen spectrum is 300-400° C.
 15. A liquid crystal display panel inaccordance with claim 9, wherein the first conductive film is connectedto at least one of the first signal lines or the second signal lines.16. A liquid crystal display panel in accordance with claim 9, whereinthe second conductive film is connected to the pixel electrodes.
 17. Aliquid crystal display panel in accordance with claim 9, wherein thesecond conductive film comprises a first portion and a second portion.18. A liquid crystal display panel in accordance with claim 17, whereinthe first portion and the second portion of the second conductive filmare formed apart from each other.
 19. A liquid crystal display panel inaccordance with claim 17, wherein the first portion is connected to atleast one of the first signal lines or the second signal lines.
 20. Aliquid crystal display panel in accordance with claim 17, wherein thesecond portion is connected to the pixel electrodes.
 21. A liquidcrystal display panel in accordance with claim 10, wherein the pixelelectrodes and the second conductive film comprise a single film.